Vehicles with manual transmissions and the motor turned off can generally be secured from rolling away by way of a parking brake, and additionally by way of a gear, having with a high transmission ratio, being inserted into the transmission and locking an output of the drive train. In vehicles, the drive trains of which are configured with a hydrodynamic torque converter or a hydrodynamic clutch, this possibility does not apply since, with a turned off motor, a connection between the motor, with its braking power, and the drive cannot be maintained. In order to provide a safety feature against rolling away, even under extreme conditions, vehicles with hydrodynamic torque converters or hydrodynamic clutches are equipped with a separate device, which is normally integrated in the transmission as a parking brake, to secure the output.
Parking brakes of the type currently known in practice prevent an unintended rolling of the vehicle even with the motor turned off. If a parking brake is activated or engaged, then a transmission drive shaft which is connected to the drive wheels of the vehicle in a rotationally fixed manner, is locked in such a way that the vehicle is secured from rolling away of its own volition on level surfaces, as well as on slopes up to approximately a 30% grade. The activation of such parking brakes occurs by way of a driver in that he places a gearshift in a park position.
One implementation of the parking brake, known in practice, is implemented with a parking brake wheel connected in a rotationally fixed manner to a transmission drive shaft, which has a type of splined shaft profile on its outer diameter. A pawl is further provided, which radially grips the splined shaft profile of the parking brake wheel, in a positive locking manner when the parking brake is activated, whereby the transmission drive shaft is then locked.
When the parking brake is released, the pawl is released from gripping the parking brake wheel by way of a mechanical, hydraulic, or electrical mechanism, so that the parking brake wheel or the transmission drive shaft is unlocked and the output of the drive train is no longer locked. The release of the parking brake also occurs depending on a corresponding control action from the driver, in that the driver repositions the gearshift out of the park position and into another desired gearshift position.
In the case of the engaged parking brake, torque is applied to the drive wheels from the pawl by way of the transmission and the output. As an example, the torque is produced when a vehicle is facing in a forward driving position, either uphill or downhill and the force of the downhill grade on the vehicle is directed away from the drive direction by the drive train. Through locking the transmission drive shaft with the parking brake, the drive train is kept secure between the drive wheels and the parking brake. When the parking brake is disengaged, the strain of the drive train, which is passed by the chassis of the vehicle above the suspension point of the drive train, is suddenly released with a slight or a strong release jolt, which is especially pronounced in the case of steep slopes, and is disadvantageous for the driver, on one hand, as an unpleasant jolt and, on the other hand, is also noticeable as an unpleasant noise. Additionally, it strains the affected components of the transmission mechanically.
From DE 102 55 714 A1 of the Applicant, a method for controlling an automatic transmission with multiple shifting elements and a parking brake for positive locking restraint of the output of a drive train is known in which, simultaneous with the engagement of the positive locking parking brake, the output is locked by at least one engaged friction locking shifting element of the automatic transmission, such that a release jolt, occurring upon the release of the positive locking parking brake, is reduced, because possible movement between the drive wheels and the output of the vehicle is prevented when the brake is disengaged since the movement of the output is locked both before and after brake disengagement by the engaged friction locking shifting element. The strain on the output is preferably eliminated through a subsequent controlled disengagement of the engaged friction locking shifting element without a noticeable release jolt, by which an output can be rotated after the disengagement of this shifting element in accordance with a driver's control action. If an automatic transmission is already provided with a shifting element for frictionally locking the output, and also necessary for gear shifting and is constructed as a multiple disc clutch or multiple disc brake in the usual manner of construction, the pressurizing medium supply of this shifting element must always be guaranteed for the engaged parking brake, which leads to problems when the drive motor of the vehicle is turned off. Alternatively, the friction locking shifting element for the restraint of a drive could equally expensively be constructed so that it engages without pressure and disengages with application of a pressurizing medium. Correspondingly, the method in DE 102 55 714 A1 is not applicable in the same manner without problems in various transmission types.
The purpose of the present invention is the making of a vehicle transmission having multiple friction locking shifting elements and a parking brake for the positive locking restraint of an output of a drive train of the vehicle, and a universally applicable method of controlling such a vehicle transmission such that a disturbing release jolt upon release or disengagement of the previously engaged parking brake is avoided or reduced, and it is no longer viewed by the driver of the vehicle as disturbing.